There are known to be numerous coating compositions having hexavalent chromium for forming a corrosion resistant coating on metals. Such chromate coating compositions are useful in providing coatings which aid in the adhesion of subsequently applied siccative organic finishes. Should an organic finish be applied to such a chromate conversion coating, the organic finish can become discolored, particularly if it is of a light color. The discoloration is caused by a "bleeding" of soluble hexavalent chromium salts from the coating into the organic finish. Attempts have been made to reduce such discoloration in several ways. The most common method of eliminating the soluble hexavalent chromium salts from this coating is by rinsing. When a rinse is employed to remove the hexavalent chromium salts from the coating, a buildup of the soluble salts can occur in the rinse bath. To prevent the build-up of soluble chromate salts in a rinse bath, it is necessary to continually overflow the bath or discard the rinse solution once it is used.
Because of the problems associated with hexavalent chromium in the conversion coating, attempts have been made to modify the coated metallic surfaces. In some attempts to modify the coating, rinse compositions have been employed. Compositions having trivalent chromium have been used as a final rinse subsequent to contacting the metallic surface with a conversion coating composition. Such rinses have been found to aid the corrosion resistance and paint adhesion characteristics of previously coated metallic surfaces. For example, metal surfaces have been rinsed with a chromium final rinse composition, wherein a portion of the hexavalent chromium had been reduced to the trivalent state, by acids, aldehydes or alcohols.
Examples of such attempts are known in U.S. Pat. Nos. 3,063,877; 3,222,226; and 3,279,958.
Other attempts to solve the problems caused by the presence of hexavalent chromium in the conversion coating have been made by reducing the hexavalent chromium to the trivalent state in the coating itself after it is formed on the metal surface. Such procedures are found, for example, in U.S. Pat. Nos. 3,094,441 and 3,535,168 and British Pat. No. 1,114,645.
Such attempts resulted in decreasing the amount of soluble hexavalent chromium on the coated metal surface, but at the expense of contributing toxic compounds to the effluent of the process.
Effluents containing chromium have been found to be toxic, particularly when the chromium is in the hexavalent state. It is desirable to eliminate or diminish the amount of chromium and especially hexavalent chromium in the effluent. It has long been desirable to eliminate the rinse for reasons of the savings that could be realized in the cost product and apparatus. It is now desirable to prevent the discharge of harmful effluent of a conversion coating process for manifest reasons of environmental concern.
It is an object of this invention to produce a corrosion resistant coating on a metallic surface. It is another object of the invention to provide a coating solution and process which requires no subsequent rinsing. It is a concomitant object of the invention to provide process having a closed system for coating metal surface which eliminates effluent of the process. It is another object of the invention to provide a coating process which can be operated at room temperature. It is another object of the invention to form a conversion coating which is adherent of a subsequently applied siccative finish. Still other objects of the invention shall become evident from the description which follows.